In pulp treating processes, pulp may be washed and dewatered in order to raise its consistency for a subsequent treatment stage. In some processes, the pulp is in the form of a web and may be treated in steps. In twin wire presses, the pulp web to be dewatered is located between two opposed wires which are pressed towards each other by means of perforated “tables” (plates/foils) and subsequent press rolls, such that the web may be dewatered through the permeable wires and further through the surface of the plates. Twin wire presses are commonly used for dewatering of pulps with an inlet consistency of approximately 3-10%. Pulp to be dewatered in twin wire presses is often pulp produced in a mechanical pulping process.
Conventionally, in a twin wire press for handling pulp, plates/foils and press rolls are combined although separated, and often a table dewatering section is followed by a press roll arrangement such that the wires pass directly from the table dewatering section to a series of nips formed between opposed press rolls. This arrangement is advantageous because in a nip formed between two press rolls the pressure may be much greater than in a table dewatering section. Hence, a first dewatering step is accomplished in a table dewatering section between two tables, wherein a second, more intense, dewatering is performed in a subsequent press roll arrangement involving at least two press rolls forming a nip between them. The table dewatering section is arranged in such a way that a wedge-shaped dewatering zone is created between the tables.
In a table dewatering section, the pressure between the tables may not go above a certain level, in order to prevent the wires from slipping with respect to each other. If the pressure between the tables becomes too high, a higher load on the drive roll is required which leads to higher friction. The higher friction may cause slipping in the drive roll nip which may cause the wires to slip with respect to each other in the table section. In the subsequent press roll arrangement, on the other hand, the risk of the wires slipping is minimal, especially since the wires rotate with the press rolls. Instead, the limitation in a press roll nip is that the dewatering degree may not be put too high in order not to scatter the pulp web. If the pressure between the press rolls is too high in comparison to the water content in the web, there will not be room for all of the water to be removed through the wires. Instead, part of the water will be pushed backwards through the web, such that the fiber web network may be broken, which will lead to subsequent delamination of the fiber web once outside of the nip (on the other side of the nip). The higher the pressure, the more influential the backward flow will be and the higher the risk of separation of the web. Therefore, conventionally, the first press roll nip in the press roll arrangement has to be arranged such that the risk of breaking the pulp web network is minimised, i.e. with a relatively low pressure and a low degree of dewatering. In fact, the first press nip of the press roll arrangement is therefore generally not very efficient. Instead, the object of the first press nip is to reduce the water content of the nip sufficiently such that a more efficient dewatering may be accomplished in the following press roll nip(s).
A problem with conventional dewatering table sections is hence that the dewatering achieved in them is too low, such that e.g. the first following press roll nip is dedicated to adapting the water content of the pulp web instead of efficiently decreasing it. In fact, this means that the first press roll nip is not efficiently used. This is a problem, since each press roll pair constitutes a large investment that should be utilised in full to bear its own costs.
Swedish Patent Specification No. 527,778 discloses an apparatus in which a press roll is arranged at the back end of the dewatering table section to provide an additional pressing force to improve the dewatering capacity of the arrangement. This arrangement includes a clear advantage over the prior art with respect to the dewatering degree. However, there are some limitations regarding the flexibility of this arrangement. In Swedish Patent Application No. 527,778 it is possible to adapt the force by moving the press roll towards or away from the opposed press table. In this respect the arrangement resembles a conventional press roll nip, including the risk of breaking the pulp web fiber network.
Also, the arrangement of Swedish Patent Application No. 527,778 allows little flexibility with respect to the possibility of adapting the dewatering degree of the press nip in accordance to the incoming consistency of the pulp. If the consistency of the incoming pulp increases from a given consistency to a somewhat higher consistency, i.e. the water content of the pulp increases, the force of the press roll needs to be released in order not to break the pulp web fiber network. Such an adjustment also reduces the dewatering degree of the press nip markedly, such that the possible dewatering degree of the nip between the press roll and the dewatering table becomes strongly dependent of the consistency of the incoming pulp, with little possibility to maximise the dewatering.
Therefore, there is a need of further ameliorating the dewatering degree of the table dewatering section arrangement, e.g. such that one pair of at least two following press roll pairs may be omitted. Alternatively, instead of omitting a pair of press rolls, the consistency out of the press may be increased. Another alternative which is enabled once the dewatering degree of the table dewatering section is ameliorated according to the present invention, is to ease the load on the table section which leads to less friction from the tables. It is desirable to be able to adapt the dewatering degree to a greater extent regardless of the consistency of the incoming pulp.
One object of the present invention is to provide apparatus and a method that enables an improved dewatering in a table dewatering section of a twin wire press.